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Date: 2001-11-06 11:59:30

"A cluster is a group of application servers that transparently
run your (J2EE) application as if it were a single entity. To scale, you
should include additional machines within the cluster. To minimize downtime,
make sure every component of the cluster is redundant. "

Clusters are composed of multiple machines that more or less know about
each other. This knowledge is not always equal. Some Clusters are just
a redirection of a request to a less loaded machine that can do the work.
Some other clusters share application memory and transactions.

Redundancy and failover

In addition to machines, clusters can comprise redundant and failover-capable:

  • Load balancers: Single points of entry into the cluster and traffic directors
    to individual

  • Web or application servers

  • Web servers

  • Gateway routers: Exit points out of an internal network

  • Multilayer switches: Packet and frame filters to ensure that each machine
    in the cluster receives only information pertinent to that machine

  • Firewalls: Cluster protectors from hackers by filtering port-level access
    to the cluster and internal network

  • SAN (Storage Area Networking) switches: Connect the application servers,
    Web servers, and databases to a backend storage medium; manage which physical
    disk to write data to; and failover

  • Databases

Scalability and High Availability (HA).

Regardless of how they are implemented, all clusters provide two main
benefits: scalability and high availability (HA).

  • Scalability refers to an application's ability to support increasing
    numbers of users. Clusters allow you to provide extra capacity by adding
    extra servers, thus ensuring scalability.

  • High availability can be summed up in one word: redundancy. A cluster
    uses many machines to service requests. Therefore, if any machine in a
    cluster fails, another machine can transparently take over.

Source : http://www.javaworld.com/javaworld/jw-02-2001/jw-0223-extremescale.html

Last edited on Saturday, December 31, 2005 at 13:32:23 pm.